http://www.leagueoflegends.com/This is an incredibly addictive game based on DotA. There's a shitton of characters to pick from all with differing abilities and roles. The basic idea is that there's a base in the bottom left and top right corners of the map for each team with three lanes connecting them (bottom, mid, top). Basic idea is to push up to the opposing base. Also contains elements of leveling and gear making to help you further customize how you work.
I think my favorite part is the huge variety of champions. There's somewhere around 65 currently and one is released every few weeks. The way the game works is that there's a handful of champions deemed "free" each week meaning you can play them even if they aren't unlocked. You earn IP (Influence Points) through playing games which you can save up to permanently unlock champions for varying amounts. Further customization comes from gaining mastery points and rune slots as well as Summoner Spells (you are defined as the summoner and control your champion).
MotK Grand List of Nicks:
Likaros (YellowYoshi1)
DefinitelySatori (SomewhatMystia)
DanDanRevolution (Cyclone)
ReimuHakureiSM (Golden PX)
Wasabikun (Bitmap)
Kyo Tanaka (Kyo Tanaka)
Cheez Bix (Andrewv42)
Ryuumasken (Ryuu)
Ruffa (Rukaroa)
Daxarian (Evies)
Moltentruros (Molten) (EU Server)
FFMaster (FFmaster)
Esifex (Esifex)
Fizzleboom (Cadmas)
MoltenTruros (Molten)
Caution: This game will induce rage and addictions will develop.